I siphoned mine out when they were small but if they are too big for that there is a method where you cut the top off a plastic drinks bottle and reverse it and put it back in the lager part, put some food in the lager part and the fish swim in but find it hard to get out, they may be a demo on Youtube of how to do it.
I used the old soda bottle fish trap method and it worked well for catching one of my wrasse that was stuck in the overflow.